This feature is also same as the one present in Snapchat. Essentially, in Snapchat, a user will be notified whenever someone takes a privately shared photo. And, in Instagram, the same thing is happening. Here's how this feature works in Instagram.

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Only Works with Disappearing Photos

As said earlier, Instagram recently added a new feature which allows you to post disappearing photos. So, if someone captures a screenshot of your posted disappeared photo, then you will be notified as the screenshot has taken.

Only Happens With the Latest Update

Also, the notification has received by the people whoever updated their mobile application with the latest one. That being said, users without the latest update are just fine.